Abstract

The alpha adrenergic control of a propagating vascular smooth muscle, the isolated portal vein of the rat, was analyzed. The purpose of the study was to investigate whether a single model of the receptor-agonist reaction would quantitatively account for responses both to nerve stimulation and to exogenous norepinephrine (NE). The dissociation constant of the receptor-agonist complex was found to be 1.26 x 10-6 M. Receptor kinetics and an expression relating fractional receptor occupancy to response gave a theoretical dose-response curve for exogenous NE which was consistent with the experimental resuits. Transmitter concentrations at the receptors, 1000 å from the sites of neural release, were calculated by applying the diffusion equation and were found to be characterized by peaks exceeding 10-5 M. Theoretical frequency-response curves could be calculated by considering the qualitative properties of these transmitter peaks and the agonist receptor relation developed for exogenous NE. Consistency between theoretical and experimental frequency-response curves was obtained under conditions where the NE peaks caused high levels of initial receptor occupancy. This suggests that a single receptor-agonist reaction can determine alpha adrenergic responses of this neuroeffector both to exogenous NE and to nerve stimulation despite the obvious difference in the distribution of the agonist.

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